The temperature of integrated circuit chips must be kept below specified limits to ensure proper function, reliability and useful life. The trend in integrated circuit technology is to pack more circuits per chip which increases the heat generation per chip. Also, system designers are mounting chips closer together to minimize propagation delays in the interconnections. These trends and designs have increased heat flux, i.e. power per unit area and caused a need for improved cooling techniques.
In the cooling of heat producing elements, a conductive heat transfer medium (a solid) is placed into contact with a heat producing element. The medium either has, or contacts another element with has, a greater surface area relative to the heat producing element so that heat is more easily dissipated from the greater surface area. To enhance heat dissipation from surface areas, a fluid is often used as a heat transfer medium by being moved over the heat dissipating surface area to "carry away" heat by convection. From the foregoing it becomes quite clear that heat transfer is enhanced when there is greater surface contact between a heat producing element and a heat transfer medium.
The development of multichip thermal conduction modules to enhance the cooling of concentrations of chips resulted in various conduction cooling techniques including a plurality of resiliently urged pistons each contacting a chip and providing a thermal path to a portion of the module housing which is convection cooled by a fluid coolant.
This technique was further enhanced by encapsulating the pistons in Helium gas to promote conduction cooling. Also, coolants such as air, water and fluorocarbons have been pumped through the housings.
One limitation of fluorocarbons used in immersion cooling is that they chemically break down after prolonged use. Another limitation is that if the fluorocarbon coolant has been permitted to contact the chips, contamination can occur.
Such pistons limit heat transfer regardless of piston geometry due to the rigidity of the piston. For example, if the piston has a curved contact surface then limited point contact with the relatively planar chip surface results in reduced heat transfer. Where the piston also has a relatively planar contact surface, the piston and chip contact surfaces must be in substantial alignment to avoid point contact.
Another approach, as an alternative to the resiliently urged pistons, was to use a flexible microbellows as a conductive heat transfer medium to increase contact surface area between the conductive heat transfer medium and the chip.
In the basic microbellows cooling technique, a flat contacting surface of the microbellows, usually about 0.0015 inches thick of Nickle, contacts a flat surface of the chip. By design, the microbellows are flexible in axial and angular directions. However, the contacting surface of the microbellows requires high pressures for conforming to the chip surface when the contacting surface area of the microbellows are smaller or equal to the chip surface area. If the surface area of the microbellows is larger than the chip surface area, which is more desirable, the microbellows may make contact at the four corners of the chip surface area. This imposes high heat resistance to heat flow.
The foregoing illustrates limitations known to exist in present devices. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.